Clarify the association between the APIs in the PQC Extension and the main spec

[athoelke]

The sections of chapter 2 in the PQC extension do not refer directly to the relavent part of the Crypto API that they are to be used with.

This might be obvious for the hash and signature algorithms, as these have always been present in the Crypto API, but not for ML-KEM, which uses the key-encapsulation API that was only introduced in v1.3, concurrent with the publication of the PQC Extension.